What's actually good for me to spend resin on at AR25, now that I've finished up the event?
I would be focused on planning out your ascension materials. If you're situated for your current level, start preparing your team for 30 by killing the elemental bosses (cubes, etc) and running weapon domains. If you have all those covered, you can look into talent materials, too. Don't forget talents exist!
But don't blow resources on things you don't need. It's easy to fill like "it'd be nice" to start improving a bunch of other characters, and that's a Bad Idea (tm). You will need those resources. When in doubt, you can go always go back to leylines, though I wouldn't do the blue ones until they start giving you Hero's Wit.
That makes sense, good to know. ---
"Let's all go play nagasaki. We can all get vaporized. Hold my hand, let's turn to ash. I'll see you on the other side."